If former House Speaker Newt Gingrich's past statements are to be believed, the primary threat facing the U.S. isn't a foreign adversary but the gradual imposition of Islamic Sharia law in America.

Gingrich likes to couch his policy positions in broader ideas, invoking his training as an historian to offer context and clarity. So it went in a 2010 speech to the American Enterprise Institute. He depicted a clash of civilizations and ideologies between western liberalism and radical Islam, which he likened to World War II in its importance and the Cold War in its duration.

Gingrich's View of Sharia's Status in U.S.

But Afghanistan, the current presidential frontrunner asserted at the time, is not the central front in this conflict. It is in the United States, where Gingrich contended Sharia principles were slowly and insidiously taking hold.

"Stealth jihadis use political, cultural, societal, religious, intellectual tools; violent jihadis use violence," Gingrich said. "But in fact they're both engaged in jihad and they're both seeking to impose the same end state, which is to replace Western civilization with a radical imposition of Sharia."